The mechanistic target of rapamycin complex 1 (mTORC1) protein kinase is a master growth regulator that senses diverse environmental cues, such as growth factors, cellular stresses, and nutrient and energy levels. When activated, mTORC1 phosphorylates substrates that potentiate anabolic processes, such as mRNA translation and lipid synthesis, and limits catabolic ones, such as autophagy. mTORC1 dysregulation occurs in a broad spectrum of diseases, including diabetes, epilepsy, neurodegeneration, immune response, suppressed skeletal muscle growth, and cancer among others (Howell et al., (2013) Biochemical Society transactions 41, 906-912; Kim et al., (2013) Molecules and cells 35, 463-473; Laplante and Sabatini, (2012) Cell 149, 274-293).
Many upstream inputs, including growth factors and energy levels, signal to mTORC1 through the TSC complex, which regulates Rheb, a small GTPase that is an essential activator of mTORC1 (Brugarolas et al., (2004) Genes & amp; Development 18, 2893-2904; Garami et al., (2003) Molecular Cell 11, 1457-1466; Inoki et al., (2003) Genes & amp; Development 17, 1829-1834; Long et al., (2005) Current Biology 15, 702-713; Sancak et al., (2008) Science (New York, N.Y.) 320, 1496-1501; Saucedo et al., (2003) Nature cell biology 5, 566-571; Stocker et al., (2003) Nature cell biology 5, 559-565; Tee et al., (2002) Proc Natl Acad Sci USA 99, 13571-13576). Amino acids do not appear to signal to mTORC1 through the TSC-Rheb axis and instead act through the heterodimeric Rag GTPases, which consist of RagA or RagB bound to RagC or RagD, respectively (Hirose et al., (1998) Journal of cell science 111 (Pt 1), 11-21; Kim et al., (2008) Nature cell biology 10, 935-945; Nobukuni et al., (2005) Proc Natl Acad Sci USA 102, 14238-14243; Roccio et al., (2005) Oncogene 25, 657-664; Sancak et al., (2008) Science (New York, N.Y.) 320, 1496-1501; Schürmann et al., (1995) The Journal of biological chemistry 270, 28982-28988; Sekiguchi et al., (2001) The Journal of biological chemistry 276, 7246-7257; Smith et al., (2005) The Journal of biological chemistry 280, 18717-18727). The Rag GTPases control the subcellular localization of mTORC1 and amino acids promote its recruitment to the lysosomal surface, where the Rheb GTPase also resides (Buerger et al., (2006) Biochemical and Biophysical Research Communications 344, 869-880; Dibble et al., (2012) Molecular cell 47, 535-546; Saito et al., (2005) Journal of Biochemistry 137, 423-430; Sancak et al., (2008) Science (New York, N.Y.) 320, 1496-1501). Several positive components of the pathway upstream of the Rag GTPases have been identified. The Ragulator complex localizes the Rag GTPases to the lysosomal surface and, along with the vacuolar-ATPase, promotes the exchange of GDP for GTP on RagA/B (Bar-Peled et al., (2012) Cell 150, 1196-1208; Sancak et al., (2010) Cell 141, 290-303; Zoncu et al., (2011) Science Signaling 334, 678-683). The distinct FLCN-FNIP complex acts on RagC/D and stimulates its hydrolysis of GTP into GDP (Tsun et al., 2013). When RagA/B is loaded with GTP and RagC/D with GDP, the heterodimers bind and recruit mTORC1 to the lysosomal surface, where it can come in contact with its activator Rheb GTPase.
Recent work has identified the GATOR1 multi-protein complex as a major negative regulator of the amino acid sensing pathway and its loss causes mTORC1 signaling to be completely insensitive to amino acid starvation (Bar-Peled et al., (2013) Science 340, 1100-1106; Panchaud et al., (2013) Science Signaling 6, ra42). GATOR1 consists of DEPDC5, Npr12, and Npr13, and is a GTPase activating protein (GAP) for RagA/B. The GATOR2 multi-protein complex, which has five known subunits (WDR24, WDR59, Mios, Sec13, and Seh1L), is a positive component of the pathway and upstream of or parallel to GATOR1, but its molecular function was, until recently, unknown (Bar-Peled et al., (2013) Science 340, 1100-1106).
Recently, additional information about the mTORC1 pathway has been elucidated by identifying the binding of GATOR2 with one or more of the Sestrins and demonstrating that the resulting Sestrin-GATOR2 complex regulates the subcellular localization and activity of mTORC1. In particular, the presence of Sestrin-GATOR2 complexes inhibits the mTORC1 pathway and decreases mTORC1 activity by preventing translocation of mTORC1 to the lysosomal membrane. Interaction of GATOR2 with the Sestrins, and in particular Sestrin1 and Sestrin2, is antagonized by amino acids, particularly leucine and, to a lesser extent, isoleucine, methionine and valine. In the presence of leucine, GATOR2 does not interact with Sestrin1 or Sestrin2 and mTORC1 is able to migrate to the lysosomal membrane where it is active. Sestrin1 and Sestrin2 directly bind leucine and to a lesser extent, isoleucine and methionine (Chantranupong et al., (2014) Cell Rep.; 9(1):1-8). The binding of leucine by Sestrin1 or -2 is required for disruption of its interaction with GATOR2 and subsequent activation of mTORC1. Sestrin2 mutants incapable of binding leucine cannot signal the presence of leucine to mTORC1, and cells depleted of Sestrin2 and its homologs render mTORC1 insensitive to the absence of leucine (Wolfson et al., (2015) Science pii: ab2674 [Epub ahead of print]).
The Sestrins are three related proteins (Sestrin1, -2 and -3) of poorly characterized molecular functions (Buckbinder et al., (1994) Proc Natl Acad Sci USA 91, 10640-10644; Budanov et al., (2002) Cell 134, 451-460; Peeters et al., (2003) Human genetics 112, 573-580). Sestrin2 inhibits mTORC1 signaling and has been proposed to activate AMPK upstream of TSC as well as interact with TSC (Budanov and Karin, (2008) Cell 134, 451-460), but later studies find inhibition of mTORC1 by Sestrin2 in the absence of AMPK (Peng et al., (2014) Cell 159(1):122-33) further emphasizing the important role the GATOR2 complex plays in modulating mTORC1 in response to Sestrin2.
Modulation of the Sestrin-GATOR2 complex represents a potential therapeutic target for selectively modulating mTORC1 activity indirectly.